Polar Bear Research Paper

The distinct evolutionary relationship and hybridization of brown bears (Ursus arctos) and polar bears (Ursus maritimus).

Fast morphological evolution has been seen over the years throughout mammals of the earth, as it is an important driver of specialization and speciation. Ursus, is the genus in the family of Ursidae (bears) that includes polar bears (Ursus arctos), black bears (Ursus americanus), and brown bears (Ursus maritimus). The polar bear and brown bear specifically are closely related. In fact, there are estimates that polar bears branched off from brown bears during the mid to late Pleistocene, becoming isolated on Siberian costal enclaves, an Arctic sea ice environment. (C. Lindqvist
et al. 2010) For this reason it is ironic that the two species have successfully been able to produce hybrids not only with human influences, but also within the wild. This paper will describe proof of the distinct evolutionary lineage and recent hybridizations of polar bears and brown bears. The polar bear is the only ursid that seems to be a uniquely adapted high-arctic specialist.
(Hailer et al. 2012) They roam the Arctic ice sheets, swim in coastal regions and have a fully carnivorous diet of seal flesh and blubber. Molecular and paleontological data indicate that the polar bear evolved less than a million years ago from a distinct other ursid, the brown bear. A brown bear has a complete omnivorous diet. (Slater et al. 2010) This is because they live in a completely different geographical region than polar bears. Brown bears can be found throughout mountains and forests of the Holarctic and in the Tundra. (PreuB et al. 2009) G. Slater et al. (2010) looked at dietary specializations using phylogenetic tests to estimate the rate that morphological specialization occurred in polar bears. It was observed that very fast evolution of semi-aquatic adaptations and dietary specialization produced cranial morphology that is weaker than that of brown bears primarily because of their different diets. The mean rate which was calculated for an ursid was rate= 0.024, +/- 0.007. The rate for a polar bear was, 0.059. This means that the rate of skull shape evolution was about double the mean that was observed for other parts of ursid phylogeny thus proving fast morphological evolution. (Slater et al.
2010)

Genetic studies have shown that polar bears are more closely related than another other brown bear to a unique clade of brown bear populations that live directly on the Admiralty, Baranof, and Chichagof (ABC) islands of southeastern Alaska’s Alexander Archipelago.
Fossil remains of polar bears are hard to find. This is because when majority of them die they are likely to be over sea ice, or scavenged by other animals. (C. Lindqvist et al. 2010). C. Lindqvist et al. (2010) also writes about a lower jawbone that is said to be the oldest polar bear remains ever found. Accelerator mass spectrometry was used to determine the age of a canine tooth left on the jaw. It was estimated to be approximately 130-110 ky old. It is said that this jawbone confirms that the polar bear was already a species at least 100 ky ago and was preserved due to the low temperatures and permafrost that was present during that time. Tests such as “next generation” sequencing technology and isotopes analyses of carbon and nitrogen of past and present environments were used. The date of a split between the brown bear and polar bear was concluded to be a mean of 152 ky. Isotope data collected also proved that the polar bear morphologically and physiologically adapted at a very rapid
pace.

Studies using mitochondrial DNA with nuclear markers have also been used to estimate the time in which the polar bears evolved from brown bears, most of which have been ineffective. To make an effective estimate Hailer et al. (2012) had to test a diverse number of nuclear sequences of polar bears and brown bears. Using many unlinked nuclear loci allows analysis of inherited genetic markers, which have statistically independent information essential for seeing an unbiased picture of the evolutionary relationship between the two. Hailer et al. (2012) looked at 9116 nucleotides from 14 loci across the genome in 45 polar bears, brown bears and black bears. The results revealed that polar bears indeed have closest lineage to the ABC brown bears. In fact, some of the recently diverged species still share many of the same alleles due to polymorphisms that have been retained throughout the years.

The most recent evolution between brown bears and polar bears derive by the two species mating themselves. Hybridization as such, is not expected in nature, mostly because of the two different geographical regions in which they live in. The interbreeding between the polar bear and brown bear has also occurred several times in zoos around the world. Although the two species live in different regions, their habitats overlap making it possible for hybrids to occur successfully. (A. PreuB et al. 2009) With climate change and global warming is occurring at an exponential pace and is highly expected that in the future more polar bears will be found more inland. This means as the temperatures raise across the world, brown bears are likely to move northward. Since both species will be moving towards each other, there is a higher chance for hybridization to occur within the future. A. PreuB et al. (2009) focuses on the hybrid-bear behaviour and phonotypical differences between the hybrid and the two species. A number of characteristics were observed including back definition, length of neck, full body size, ears, hair structure and location of paw hair were all compared. The swimming behaviour and sleeping behaviour was closely observed as well. It is concluded that the bears were intermediate with swimming abilities and showed similar sleeping behaviours as both species.

In conclusion, the evolutionary relationship between brown bears and polar bears date back thousands of years. Dates are estimated throughout many studies but since the remains of polar bears are hard to find, no one will ever know an exact date. There are also multiple studies that prove that polar bears evolved from their closest ancestor, the brown bear. Most recently, polar bear-brown bear hybrids are not thought to be possible in nature as they live in two different geographical regions, but since fast occurring climate change the two species’ habitats are beginning to overlap. This means that hybrids are occurring more often in nature as well as by human influence in zoos. Thus, although there is proof of evolution of polar bears and brown bears that dates back years ago, evolution of the two species is also occurring in front of our eyes.

Bibliography

C. Lindvist et al. (2010). Complete mitochondrial genome of a Pleistocene jawbone unveils the origin of polar bear. PNAS. Vol. 107. 11

F. Hailer et al. (2012). Nuclear Genomic Sequences Reveal that Polar Bears are an Old and Distinct Bear Lineage. Science. Vol 336. 344-347

A. PreuB, U. GansloBer, G. Purschke, U. Magiera. (2009). Bear-hybrids: behaviour and phenotype. Zool. Garten N.F 78. 204-220.

G. Slater et al. (2010). Biomechanical Consequences of Rapid Evolution in the Polar Bear Lineage. PLoS ONE. Vol. 5. 11. 1-6